Lysosomal enzymes require an acidic pH to degrade engulfed material. We show here that the lysosomal pH of RPE cells from ABCA4-/- mice is elevated as compared to age matched controls. As the ABCA4 gene is mutated in Stargardt's disease, and as accumulation of partially degraded material in and around RPE cells is a key characteristic of the disease, we hypothesize that restoring an acid pH to the lysosomes will be beneficial. High-throughput screening indicates that lysosomes of perturbed RPE cells in culture are acidified by cytoplasmic cAMP, and by agonists which stimulate receptors coupled to the Gas protein. Elevation of cAMP increases the clearance of outer segments by RPE cells, demonstrating an improvement in function. This proposal investigates the contribution of cAMP, the Cl- channel CFTR, and the vesicular proton pump vH+ATPase to the restoration of lysosomal acidity. It will determine the relationship between age, A2E and lysosomal pH in RPE cells from ABCA4-/- mice and confirm that cAMP restores lysosomal acidity the these RPE cells using both in vitro and in vivo treatment strategies. PUBLIC HEALTH RELEVANCE: Stargardt's disease is an early-onset form of macular degeneration characterized by the accumulation of partially degraded material in and around RPE cells. This proposal will explore a newly identified defect in the lysosomal pH of RPE cells from the ABCA4-/- mouse model of Stargardt's disease that may slow processing of photoreceptor outer segments, develop treatments to restore this defect, and determine how these treatments work.